Measurements of acoustic impedance in, e.g., different acoustic loads are of interest in many branches of acoustics, including hearing diagnostics, acoustic muffler systems, and musical acoustics. These measurements are typically carried out with an impedance probe comprising an acoustic transducer, such as a receiver (generally also known as a speaker), delivering a stimulus (e.g., an input signal) to an applied acoustic load and an acoustic energy detector, such as a microphone of the probe, recording the reflected response. With a set of predetermined calibration parameters (such as the probe Thevenin calibration parameters) describing the source characteristics of the probe, the acoustic impedance of the acoustic load can be calculated from the probe response.
However, due to, e.g., physical differences in the coupling between the impedance probe and the acoustic load, errors causing artefacts in the impedance measurements may influence the impedance estimates calculated from the probe response. Artefacts causing errors in the impedance measurement includes, e.g., evanescent modes caused primarily by physical differences between the impedance probe and the acoustic load, for which the impedance should be measured by the probe.
Evanescent modes arise as a consequence of an acoustic volume velocity (i.e., stimuli) being injected into a waveguide across a limited part of an input plane thereof, exciting higher-order, non-propagating, evanescent modes. That is, the impedance probe used for impedance measurements of, e.g., an acoustic load necessarily has a smaller diameter than that of the acoustic load, to which the impedance probe is inserted. This results in evanescent modes being excited in the waveguide (i.e., the acoustic load for example being an ear canal) in addition to the propagating plane wave of the acoustic load, and consequently introduces errors in the impedance measured by the probe. Thus, the sought parameter, identified as the plane-wave impedance of the acoustic load, is often measured as a superposition of the actual plane-wave impedance and an unwanted non-plane-wave impedance of the acoustic load.
Calculating reflectance (i.e., reflection coefficient) from the measured impedance requires knowledge of the characteristic impedance of the acoustic load and the impedance measure, and the calculation of reflectance is therefore also affected by the errors caused by evanescent modes. Furthermore, the characteristic impedance of the acoustic load is closely related to the cross-sectional area of the acoustic load, which when performing measurements in, e.g., an ear canal is unknown. When investigating an acoustic load of unknown characteristic impedance, such as, e.g., the ear canal, the acoustic load is often assumed to have a specific, predefined characteristic impedance. This assumption introduces errors in the reflectance probe measurements caused by mismatches between the assumed and actual characteristic impedance experienced by the travelling wave down the length of an acoustic load coupled to the probe.
Therefore, for providing accurate measures of, e.g., reflectance and/or impedance, it is important that the characteristic impedance and/or the evanescent mode contribution are known. Research within the field of hearing diagnostics in acoustics has mainly focused on estimating the characteristic impedance in the ear canal, that is, several approaches regarding the estimation of characteristic impedance during in-situ measurements have been suggested. Some effort has also been put into compensating the contribution of evanescent modes in acoustic measurements without any success.
One known approach for approximating the effect of evanescent modes is to add to the impedance measure an acoustic mass as a compensation factor in series to the acoustic impedance. The acoustic mass is dependent on the diameter of the waveguide and the placement and size of acoustic input and output relative to each other. Relevant for evanescent mode compensation is thus to know and/or calculate an acoustic mass compensation factor related to the geometrical relationship between the impedance probe and the waveguide used for impedance measurements. However, this approach requires that geometrical parameters (such as the diameter) of the acoustic load in relation to the acoustic probe are known. This requirement is not always obtainable, when for example measuring acoustic impedance and sound pressure in human ear canals, where the ear canal may be considered as an acoustic load having an unknown characteristic impedance.
Other errors arising may include errors related to differences between the assumed characteristic impedance of an acoustic load in relation to the actual characteristic impedance experienced by a sound wave travelling through an acoustic load. Thus, the parameters causing errors in the impedance measurements of an acoustic load (e.g., a waveguide or a human ear canal), are dependent on knowing at least some physical characteristics (such as diameter and/or input/output relationship) of the acoustic load (e.g., a waveguide or ear canal) on which the characteristic impedance for the purpose of providing a reflectance measure should be measured. The physical characteristics of the acoustic load, for which the characteristic impedance should be estimated, are not always directly obtainable within acoustic applications.
Accordingly, no accurate method exists for determining the errors arising from evanescent modes in acoustic impedance measures. Thus, prior measurements have been affected by these errors. In addition, the prior art does not seem to take into account unknown parameters during in-situ measurements of an acoustic load (e.g., an ear canal), which would be of interest for, e.g., reflectance measurements.
Therefore, it is an object of the present disclosure to provide a method to compensate for errors arising in an impedance measure due to evanescent modes causing inaccuracies in a reflectance measure of an acoustic load having an unknown characteristic impedance, and an error arising in the reflectance measure due to the unknown characteristic impedance of the acoustic load. Furthermore, an object of the present invention is to use said compensation of the method to account for errors arising in, e.g., in-situ measurements of reflectance in the human ear canal, where the characteristic impedance of the acoustic load is unknown.